Multichip packaging approaches (MCP) or multichip modules (MCM) are known to provide significant performance enhancements over single chip packaging approaches. In MCP, several bare semiconductor chips (ICs) are mounted and interconnected on a common substrate through very high density interconnects. Advantages of this approach include a significant reduction in the overall size and weight of the package, which directly translates into reduced system size. Thus, first level advantages include:
1. Higher silicon packaging density--about a factor of 4 compared to surface mounted ICs on a printed wiring board (PWB). PA1 2. Short chip-to-chip interconnections--at least a factor of two reduction in interconnect links. PA1 3. Low dielectric constant materials--about three compared to nine for alumina and four to five for PWB materials. PA1 4. Higher wiring density. PA1 1) increased system speed. PA1 2) Increased reliability. PA1 3) Reduced weight and volume. PA1 4) Reduced power consumption (a factor of 4) for the same level of performance. PA1 5) Reduced heat to be dissipated for the same level of performance.
These benefits lead to the following secondary benefits:
While there are a number of currently utilized MCP approaches, existing MCP approaches suffer from at least one of the following two significant limitations. First, all such approaches require a hermetic package to protect the ICs. This is typically a metal or ceramic casing which encapsulates and seals the MCP/MCM to protect against both stray electrical fields and to protect it against environmental factors such as water vapor and gases.
The second limitation for most existing approaches is that the area beneath the ICs is shared both for electrical routing and heat removal. This results in some sharing of electrical and thermal paths, causing a thermal performance penalty. The thermal performance penalty results in the ICs operating at higher temperatures, thereby reducing the lifetime of the ICs. This reduces the reliability of systems in which the MCPs are utilized and increases the maintenance cost of such systems.
A need therefore exists for an improved MCP approach which achieves the electrical and environmental protection of the chips without the cost, size and weight disadvantages of an extra hermetic package and which eliminates the thermal performance penalty associated with the same area beneath the ICs being used for both electrical routing and heat removal.
Similar problems to those described above arise when packaging a single chip, when packaging microwave or RF components and when packaging optical components. A particular problem with the latter types of components is bringing output leads, for example transmission lines or optical fibers, through the side wall of the MCM package containing the optical and/or electrical devices. For example, with optical fibers, glass frits with ceramics and metal are utilized to seal around the fiber to retain the desired moisture and environmental protection. This is, however, an expensive process due to the cost of the materials as well as the labor involved. The heat involved in forming the glass seal also presents the risk of damaging electronic devices within the package during assembly. This sealing technique also has the disadvantage of making the part bulky and heavy, limiting its desirability for avionics and space applications. While attempts have been made to encapsulate fibers in ordinary polymers, such polymers do not provide a barrier to moisture, have limited structural integrity and coefficients of thermal expansion (CTE) which are high enough, and differ enough from that of the glass fibers as to create thermal cycling stresses. This reduces yields during assembly and adversely affects the long term reliability of the packages. The poor moisture barrier properties and poor mechanical properties also adversely affect the yield and reliability of the resulting packages. It is also important that the sealing technique utilized not cause mechanical stresses during packaging assembly since this can cause misalignment of fibers previously aligned, for example passively aligned, with corresponding devices. Such loss of alignment can adversely affect the performance and reliability of the device and can increase costs by adversely impacting the yield of usable devices. A need therefore exists for an improved technique for packaging devices having optical fibers or other information carrying leads exiting therefrom, which technique provides a good, thermally stable seal and moisture barrier.